The disclosure of U.S. Provisional Application No. 61/527,476, filed Aug. 25, 2011, is hereby incorporated herein in its entirety by reference.
Continuously variable transmissions utilizing a hydrostatic power unit, hereinafter sometimes referred to as hydro-mechanical continuously variable transmissions, are well known. In operation, the fluid displacement of the hydrostatic power unit is varied to vary the output to input ratio of the transmission, that is, the ratio between the rotating output of the transmission, and the input. In one category of the transmissions, the hydrostatic power unit is configured such that to effect movement of the vehicle in one direction, a swash plate of the unit will be tilted in one direction. To effect vehicle movement in the opposite direction, the swash plate is tilted in the opposite direction. When no vehicle movement is sought, e.g., no forward or rearward motion, the swash plate of the unit is moved to a zero tilt angle or near zero angle. However, when the vehicle or machine is subject to forces or loads urging movement, e.g., when on a slope or momentum is present, forces are applied which urge the swash plate to displace from the zero position. It is also known to employ some manner for holding the swash plate in position. Then, when motion is desired, the swash plate is released and tilted one way or the other to effect operation of the transmission to move the vehicle in the desired direction.
In another category of continuously variable transmissions employing a hydrostatic power unit for varying the input output ratio, the hydrostatic unit is configured such that at zero vehicle or machine speed, the swash plate of the hydrostatic power unit is at full displacement or near full displacement, in one direction or the other, depending on the range selected, direction of travel and possibly other factors. This category of transmission is known to be used in a variety of heavy vehicles such as work machines, including for construction, earth moving, forestry, and agriculture. Reference in this regard, Weeramantry, U.S. Pat. No. 7,063,638 B2, issued Jun. 20, 2006. Typically, a continuously variable hydro-mechanical transmission of this category will have a hydrostatic unit as one power input to a planetary gear set, and a mechanical connection to the engine of the machine as a second power input, with directional input controlled by directional clutches, and the output of the planetary connected via a clutch to one or more final gear reductions in connection with a load, e.g., the wheels, tracks or other drivers of the vehicle.
An advantage of continuously variable hydro-mechanical transmissions of the second type is efficiency. If the swash plate angle is zero (output speed of the hydrostatic unit is zero), the power through the hydrostatic unit is theoretically zero (it is zero, except there are losses for leakage, lubrication, cooling, etc). This is very useful if the vehicle is often operated at speeds which require low hydrostatic unit speeds. If multiple ranges are used, each range will have more efficiency in the center, when the hydrostatic unit output speed is low. Another advantage is that power is transmitted via two paths and can be split in a variety of ways. This also enables use of a smaller hydrostatic unit. This type of transmission also has an ability to tilt the swash plate of the hydrostatic unit through its full range of travel in both directions for each gear reduction or shift range. There is considerable disadvantage in the operation of the first type of transmission by having to move the swash plate angle to the opposite direction to engage reverse which is not present with the second type, as discussed below. Also, another advantage is that the forward and reverse clutches can be located on the input side of the transmission, so that the torque capacity of those clutches does not have to be as large, and those clutches therefore do not have to be as large. Another advantage is that the transmission can be symmetric, having the same number of forward and reverse ranges, and the same top speed in reverse.
An important operational distinction between the second category of transmission and the first is that for the second type, as noted above, the zero tilt angle position of the swash plate is an intermediate position within a shift range and not an end point or zero speed position. As a result, when the vehicle is at zero or near zero ground speed, the swash plate of the hydrostatic unit will be at a substantial degree of tilt in one direction or the other (depending on the directional clutch setting) in a high output condition, in contrast with the no or low output condition of the first category of transmission. Thus, when a directional change is made via the directional clutches, the swash plate must be moved through essentially its full range of travel, that is from a fully or near fully tilted position in one direction, through the zero tilt, to a fully or near fully tilted position in the opposite direction, to change the direction of the hydrostatic unit output. This takes time, which has a number of disadvantages, including that some operators may not desire to have any significant delays or pauses when shifting, for instance, when shuttle shifting. Also, if it is desired for the vehicle to not move during a directional change, some manner of holding the vehicle stationary must be employed. This can comprise, for instance, application of the brake or brakes of the vehicle by the operator. However, this requires additional operator action and attention. And, if on a grade or hillside, and/or when heavily loaded, this presents a challenge, as the forces urging the vehicle to move can be substantial, so as to require application of a heavy braking force. Initiation and release of the vehicle brakes can also cause lurching of the vehicle when on a grade or hillside, and/or heavily loaded.
Thus, what is sought is a manner of overcoming one or more of the disadvantages or shortcomings set forth above.